KINETICS OF OXYGEN DISSOCIATION ON SI(111)7X7 INVESTIGATED WITH OPTICAL 2ND-HARMONIC GENERATION

Optical second-harmonic generation is used as a sensitive, nondestructive probe for the thermally activated dissociation of molecular oxygen on Si(111)7 x 7. Isothermal measurements show that the dissociation kinetics strongly depend on the total oxygen coverage. At room temperature the 1/e lifetime of chemisorbed O2 is approximately 15 min for saturation coverage but approximately 50 min for low oxygen coverage. The temperature dependence of the dissociation rates between 80 and 500 K indicates that more than one dissociation channel and/or molecular oxygen species exist on the surface.